Method for SLS of PEKK and articles manufactured from the same

ABSTRACT

A process for layer-by-layer manufacturing a three-dimensional object from a powder and objects made from the same is described. The process includes the step of applying a layer of a powder on a bed of a laser sintering machine. The powder includes polyetherketoneketone. The process further includes the step of solidifying selected points of the applied layer of powder by irradiation using heat energy introduced by a laser having a power L and successively repeating the step of applying the powder and the step of solidifying the applied layer of powder until all cross sections of a three-dimensional object are solidified. L is between 1 W and 20 W. In some embodiments L is between 1 and 10 W. In some embodiments, the powder is recycled PEKK powder. In some embodiments, the powder includes carbon fiber.

TECHNICAL FIELD

The present disclosure generally relates to additive manufacturingtechnology and techniques. More specifically, the present disclosurerelates to a system and method for determining an optimal setting for alaser in a selective laser sintering (“SLS”) machine to additivelymanufacture (i.e., print) an object from polyaryletherketone (“PAEK”)polymer powder, recycled powder, and/or other materials.

BACKGROUND

It is known to use additive manufacturing technology and techniques,together with polymer powders, to manufacture high-performance productshaving applications in various industries (e.g., aerospace, industrial,medical, etc.).

SLS is an additive manufacturing technique that uses electromagneticradiation from a laser, for example from a CO₂ laser, to selectivelyfuse particles of plastic, metal (direct metal laser sintering),ceramic, or glass powders into a mass having a desired three dimensionalshape. SLS may also be referred to as laser sinter or LS. The mass orobject is built layer-by-layer on a base plate with the laser traversingeach layer in the x-y plane. The laser selectively fuses the powdermaterial by scanning cross-sectional layers generated from athree-dimensional digital description of the desired object onto the toplayer or surface of a bed of powder material. That is, the powder mustabsorb enough laser energy to reach a fusing state necessary for bondingbetween powder particles. After a cross-sectional layer is scanned, thebase plate is lowered by one layer thickness in a z-axis direction, anew layer of powder material is disposed on the bed, and the bed isrescanned by the laser. This process is repeated until the build (i.e.,building of the object) is completed.

Prior to scanning, an SLS machine may preheat the powder materialdisposed on the bed to a temperature proximate to a melting point of thepowder. Preheating may be accomplished by heating the actual bed, whichtransfers energy to the powder in the form of heat via thermalconduction. Pre-heating is also provided via radiant heaters disposedabove the bed surface. Preheating the powder makes it easier for thelaser to raise the temperature of powder to a fusing point and mayinhibit unwanted distortions in the formed object(s) during cooling.

After the layer-wise (layer-by-layer) process is completed, the formedobject(s) is disposed in a volume of unfused powder material, referredto as a cake. The formed object(s) is extracted from the cake. Thepowder material from the cake that is not fused into the built object(s)can be recovered, sieved, and used in a subsequent SLS build process.

Polyaryletherketones (“PAEK”) are of interest in the SLS process becauseparts that have been sintered from PAEK powder are characterized by alow flammability, a good biocompatibility, and a high resistance againsthydrolysis and radiation. The thermal resistance at elevatedtemperatures as well as the chemical resistance distinguishes PAEKpowders from ordinary plastic powders. A PAEK polymer powder may be apowder from the group of polyetheretherketone (“PEEK”), polyetherketoneketone (“PEKK”), polyetherketone (“PEK”), polyetheretherketoneketone(“PEEKK”), or polyetherketoneetherketoneketone (“PEKEKK”).

The SLS process is controlled by several groups of parameters, includinglaser-related parameters, scan-related parameters, powder-relatedparameters and temperature related parameters. Laser-related parameterscan involve laser power, spot size, pulse duration, and pulse frequency.Laser power, in particular, is an important build parameter in lasersintering. The density/porosity and mechanical behavior of an objectmanufactured by SLS is a function of the laser power and the accumulatedlaser energy density. In some embodiments, the power of the laser (e.g.,CO₂ laser) in the SLS machine can by adjusted prior to each build. Forexample, the EOSINT P 800 laser sintering system created by EOS Gmbh isprovided with a 50 W CO₂ laser. The P 800 laser, for example, allows foradjustments to the power of the laser beam. In regard to building partsusing PAEK polymers, in particular polyetheretherketone (“PEEK”), it isknown to use a laser power between 30 W and 50 W to perform lasersintering. Further, parts built by conventional laser sintering methods(at high laser power) have demonstrated tensile strength and/orductility that is lower than injection molded parts. The formation orpresence of porosity, regular porosity between powder layers resultingin weak planar interfaces, and other microstructural defects havelimited the use of conventional laser sintering in producing structuralcomponents.

Additionally, consistent powder characteristics are important forensuring repeatable manufacture of objects. For example, metal powdersused in additive manufacturing are assumed to be nominally spherical,and have a particle size distribution that is designed to facilitategood packing behavior, such that the final product has good mechanicalproperties. Manufacturers typically receive the base powder materialfrom third-party producers in different batches. Each batch of powdermaterial from a particular producer is assigned an identification number(e.g., batch number, lot number) for tracking and quality controlchecks. In conventional laser sintering methods, the manufacturerstypically assume when different batches of powder from the same lot areused, that the powders are identical. Working under this assumption,they apply the same laser power setting for the SLS machine in sinteringa particular powder material across different batches. However, suchassumption is not grounded in reality, as there are often batch-to-batchvariations in powder size, shape, melting points, glass transitionspoints, and other characteristics for a given powder material. This isespecially true for semi-crystalline polymers such as PEKK that exhibitmultiple melting points. Large particles tend to require more laserenergy to fuse than small particles, so large variations in size canresult in complete fusion of small particles (in one batch) andincomplete fusion of large particles (in another batch).

Thus, there exists a need in the art for an improved selective lasersintering method which utilizes lower laser power that is optimized fora given batch of powder material in order to consistently build the sameobject with greater tensile strength, as compared to the object beingmanufactured with high laser power.

SUMMARY

The needs set forth herein as well as further and other needs andadvantages are addressed by the present teachings, which illustratesolutions and advantages described below.

It is an objective of the present teachings to remedy the abovedrawbacks and issues associated with prior art selective laser sinteringmethods.

It is another objective of the present teachings to provide a method oflaser sintering a polymer powder using low laser power to build anobject(s) having sufficient tensile strength as defined by the standardtest method ASTM D638.

It is a further objective of the present teachings to provide a methodof analytically determining a laser power for laser sintering abatch/lot of polymer powder to build an object(s) having sufficienttensile strength.

It is yet another objective of the present teachings to provide a methodof analytically determining a low laser power for laser sintering abatch/lot of polymer powder to build an object(s) exhibiting sufficienttensile strength, low porosity, and minimal microstructural defects.

The above objectives and aspects of the present teachings relate to thediscovery by the inventors of new methods and systems that overcome theproblems associated with the prior art. Specifically, it has beendiscovered an analysis and examination process for determining a laserpower setting to use in an SLS machine for laser sintering a particularbatch of a given powder material to build an object(s) have greatertensile strength. The inventors have also discovered that using areduced laser power in performing laser sintering of polymer powders,including recycled powder, results in a completed part that issubstantially stronger, particularly in the Z axis, as compared to partsmanufactured with convention laser powers.

These and other objectives of the present teachings are achieved byproviding a method of analytically determining laser power for lasersintering comprising: choosing a batch of powder material for building aplurality of test rods or test coupons (e.g., dumbbell structure,dogbone structure); selecting a range of laser power for building theplurality of test rods; determining a plurality of power incrementswithin the selected range to define a plurality of different laser powersettings; programming a selective laser sintering machine to build atleast one test rod of said plurality of test rods at each laser powersetting; constructing the plurality of test rods from the batch ofpowder material based on said programming; inspecting each test rod forvoids in a surface of the test rod; identifying the laser power settingused to construct a respective test rod without formation of voids inthe surface as an optimal laser power; and configuring the selectivelaser sintering machine with the optimal laser power when conducting alaser sintering process using the chosen batch of powder material.

In some embodiments, the step of inspecting each test rod for voidscomprises inspecting an x-axis of each test rod for presence of voids.The inspection step may also take into account analyzing throughinspection each test rod's mechanical properties and tensile behaviorduring the ASTM D638 test process. For example, such inspection maycomprise inspecting a fracture point of each test rod formed during theASTM D638 test process. The method according to the present teaching mayalso comprise inspecting a porosity of each test rod. In otherembodiments, the step of inspection may involves monitoring thepresence/absence of voids in the surfaces of the test rod, the manner inwhich the test rod fractures or breaks, where the fracture occurs, thepresence of banding or bowing, and/or the porosity of the test rod. Themethod may further include steps of analyzing other factors affectingthe laser sintering process and the lifecycle of the built test rod.

Using the above inspection techniques, the optimal laser power may bedefined as at least one watt below a lowest laser power settingassociated with the formation of voids.

In some embodiments, the method also includes measuring a tensilestrength of each test rod and comparing the tensile strength results todetermine strength of each test rod. For example, the ASTM D638 testprocess may be used for measuring the tensile strength of each test rod.The measurements may include measuring the “Z strength” or break point(psi) along the z-axis for each test rod. It is known that Z strengthreflects the strength of the internal bond between the layers of anadditive manufactured part and is an important characteristic inadditive manufacturing. The measurements may also include measuring the“X strength” or break point (psi) along the x-axis. It is noted that Zstrength is a larger factor in determining whether an SLS manufacturedpart has satisfactory mechanical properties. By including tensilestrength testing, the step of identifying the optimal laser power maycomprise selecting a laser power setting based on the absence of voidsin the respective test rod and on the tensile strength of the respectiverod, wherein the optimal laser power provides the highest strength inz-axis with no formation of voids.

The step of measuring the tensile strength of each test rod may alsoinclude an analysis of the elastic modulus or Young's modulus (psi) ofeach test rod, as well as the elongation to break/fracture (%) of eachtest rod. Other factors that may be monitored and evaluated areelongation at yield, nominal strain at break (grip separation), secantmodulus of elasticity, and/or Poisson's Ratio.

In some embodiments, the step of choosing a batch of powder material totest includes choosing a batch of polymer powder material. For example,the powder material may be polyaryletherketone (“PAEK”) polymer, and insome case, more specifically polyetherketoneketone (“PEKK”) polymer. Inother embodiments, the step of choosing a batch of powder material totest includes choosing a batch of recycled polymer powder material. Therecycled powder material is left-over, unfused material from oneprevious build (“Cake A”). In some embodiments, the recycled powdermaterial is unfused material left-over after having gone through twoprevious builds (“Cake B”). Still in other embodiments, the recycledpowder material is unfused material left-over after having gone throughthree or more previous builds (e.g., “Cake C”, “Cake D”).

The present teachings also provide a method of analytically determininglaser power for laser sintering comprising: choosing a batch of powdermaterial for building a plurality of test rods having substantiallyidentical shapes; selecting a range of low laser power for building theplurality of test rods; determining a plurality of power incrementswithin the selected range to define a plurality of different laser powersettings; programming a selective laser sintering machine to build atleast one test rod of said plurality of test rods at each laser powersetting; constructing the plurality of test rods from the batch ofpowder material based on said programming; inspecting each test rod forvoids in a surface of the test rod; measuring a tensile strength in az-axis of each test rod in accordance with ASTM D638 testing andcomparing the tensile strength results to determine strength of eachtest rod; identifying the laser power setting used to construct arespective test rod having no formation of voids and high strength inthe z-axis as an optimal laser power; and configuring the selectivelaser sintering machine with the optimal laser power when conducting alaser sintering process using the chosen batch of powder material.

Exemplary ranges of low laser power to be selected may comprise 1 W to20 W, 1 W to 15 W, 5 W to 15 W, 5 W to 20 W, or any other permutation oflow laser power. In some embodiments, the laser power increments may bedetermined and set at 0.5 W. In other embodiments, the laser powerincrements may be set at 0.2 W. In yet other embodiments, the lase powerincrements may be set at 0.1 W. It is to be understood that thedisclosed increments are merely exemplary and are not limited thereto.

In some embodiments of the present teachings, the method comprisesprogramming the SLS machine to construct multiple test rods at eachpower increment using the chosen batch of powder material. With multipletest rods built using laser power at, for example 10 W, one test rod canbe subjected to Z strength testing, while another test rod is subjectedto X-strength testing. Furthermore, multiple test rods built at a givenlaser power allow for multiple tensile strength tests to be performed.That is, two different test rods built at, for example 10 W, can betested for Z strength. The multiple tests can be used for purposes ofverifying results or averaging results. Since the ASTM D638 testingprocess specifies at least five specimens for testing, it is preferablethat at least five test rods at each laser power increment isconstructed.

In some embodiments, the step of programming the SLS machine toconstruct the test rods involves programming the SLS machine to createtest rods at each laser power increment in one single build run.Accordingly, the SLS machine is able to adjust the power of the laserbeam at various points during one single build run. In otherembodiments, the step of programming the SLS machine to construct thetest rods involves more than one build to create test rods at each laserpower increment. For example, the SLS machine may be programmed toconstruct multiple test rods at a laser power of 5.0 W in a first build,multiple test rods at a laser power of 5.5 W in a second build, multipletest rods at a laser power of 6.0 W in a third build, and so on and soforth. Accordingly, the SLS machine adjusts the power of the laser atthe completion of a build. Alternatively, the SLS machine may beprogrammed to construct a first set, second set, third set, etc. of testrods in a first, second, third, etc. build, respectively, wherein eachset comprises one (or more) test rod constructed at each laser powerincrement.

The present teachings also provide a method and system for additivemanufacturing of an object comprising the steps obtaining a polymerpowder material, configuring an SLS machine to emit a laser beam at apower of between 1 W and 20 W, and laser sintering the polymer powdermaterial with the laser beam. This is in contrast to prior art, wherelaser sintering of a polymer powder is typically performed with a laserpower set in the range of 30 W to 50 W. In some embodiments, the SLSmachine is configured to emit a laser beam at a power of 5 W to 15 W. Infurther embodiments, the SLS machine is configured to emit a laser beamat a power between 5 W and 10 W, and in some case, at 8 W. The step ofobtaining a polymer power material may comprise obtaining a polymerpowder material that is recycled from a previous build. Preferably, thepolymer powder material may be unfused material left-over after oneprevious build. However, in some examples, the polymer powder materialmay be unfused material left-over after two or more previous builds. Thepolymer powder may be PAEK polymer. More preferably, the polymer powderis PEKK.

The method and system for additive manufacturing of an object may alsocomprise the step of analytically determining bed temperatures of theSLS machine, as disclosed in U.S. application Ser. No. 14/472,817, thecontents of which are incorporated herein by reference in its entiretyfor all purposes.

Other features and aspects of the present invention are furtherdescribed in the following U.S. provisional applications, each of whichis incorporated herein by reference in its entirety for all purposes:U.S. Application Nos. 62/446,470 and 62/446,460.

Other features and aspects of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate by way of example the featuresin accordance with embodiments of the invention. The summary is notintended to limit the scope of the invention, which is defined solely bythe claims attached thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an SLS machine in accordance with one embodiment ofthe present teachings.

FIG. 2 illustrates a method of analytically determining an optimal laserpower for laser sintering a particular batch of polymer powder.

FIG. 3 illustrates a qualification build layout for manufacturing aplurality of parts using SLS.

FIG. 4 is a table showing exemplary lots of powder material and theoptimal laser power determined for the respective lot.

FIGS. 5a-5d are tables showing for each lot of powder material listed inFIG. 4, the mechanical properties of a test rod built with a specifiedlaser power.

FIGS. 6a-6d are tables showing for each lot of powder material listed inFIG. 4, the x-axis inspection results of a test rod built with aspecified laser power. The data contained in FIGS. 6a-6d are associatedwith the data contained in FIGS. 5a-5d , respectively.

DETAILED DESCRIPTION

The present teachings are described more fully hereinafter withreference to the accompanying drawings, in which the present embodimentsare shown. The following description illustrates the present teachingsby way of example, not by way of limitation of the principles of thepresent teachings.

The present teachings have been described in language more or lessspecific as to structural features. It is to be understood, however,that the present teachings are not limited to the specific featuresshown and described, since the methods and systems herein disclosedcomprise preferred forms of putting the present teachings into effect.

The inventors have discovered new methods and systems to overcome theproblems associated with the prior art. Specifically, the inventors havediscovered that using a reduced laser power in performing lasersintering of polymer powders, particularly PAEK polymer powders, moreparticularly PEKK powders, and more particularly recycled PEKK powders(unfused PEKK powder left-over from one or more previous builds) resultsin a completed part that is substantially stronger, particularly in theZ axis, as compared to parts manufactured with convention laser powers.It is conventionally known to practice laser sintering of PAEK powderwith a laser power set in the range of 30 W to 50 W. The inventors,however have discovered that, unexpectedly, a laser power in the rangeof the 1 W to 10 W used to laser sinter a PEKK polymer, and morespecifically recycled PEKK polymer, produces an object having strongtensile strength and structural integrity.

In addition, the inventors have discovered a method and system todetermine a laser power setpoint for a specific lot of powder (includinga lot of recycled powder). By employing analytical methods and systemsto each lot of powder, and, in some cases, to each run of the SLSmachine, it is possible to determine an optimal laser power that resultsin finished parts having desired build properties, including moreuniformity, reduced porosity, increased tensile strength. For example,the present invention has been to shown to result in completed partshave a Z strength (i.e., tensile strength in the z direction) greaterthan 7 KSI using ASTM D638.

The present teachings is especially useful for polyetherketoneketones,or PEKK. Polyetherketoneketones are well-known in the art and can beprepared using any suitable polymerization technique, including themethods described in the following patents, each of which isincorporated herein by reference in its entirety for all purposes: U.S.Pat. Nos. 3,065,205; 3,441,538; 3,442,857; 3,516,966; 4,704,448;4,816,556; and 6,177,518. PEKK polymers differ from the general class ofPAEK polymers in that they often include, as repeating units, twodifferent isomeric forms of ketone-ketone. These repeating units can berepresented by the following Formulas and II:-A-C(═O)—B—C(═O)—  I-A-C(═O)-D-C(═O)—  II

where A is a p,p′-Ph-O-Ph-group, Ph is a phenylene radical, B isp-phenylene, and D is m-phenylene. The Formula I:Formula II isomerratio, commonly referred to as the T:I ratio in thepolyetherketoneketone is selected so as to vary the total crystallinityof the polymer. The T/I ratio is commonly varied from 50:50 to 90:10,and in some embodiments 60/40 to 80/20. A higher T:I ratio such as,80:20, provides a higher degree of crystallinity as compared to a lowerT:I ratio, such as 60:40.

The crystal structure, polymorphism and morphology of homopolymers ofPEKK have been studied and have been reported in for example Cheng, Z.D. et al, “Polymorphism and crystal structure identification inpoly(aryl ether ketone ketone)s”, Macromol. Chem Phys. 197, 185-213(1996), the disclosure of which is hereby incorporated by reference inits entirety. This article studied PEKK homopolymers having allpara-phenylene linkages [PEKK(T)], one meta-phenylene linkage [PEKK(I)]or alternating T and I isomers [PEKK(T/I)]. PEKK(T) and PEKK(T/I) showcrystalline polymorphism depending upon the crystallization conditionsand methods.

In PEKK(T), two crystalline forms, forms I and II are observed. Form Ican be produced when samples are crystallized from the melt at lowsupercooling, while Form II is typically found via solvent-inducedcrystallization or by cold-crystallization from the glassy state atrelatively high supercooling. PEKK(I) possesses only one crystal unitcell which belongs to the same category as the Form I structure inPEKK(T). The c-axis dimension of the unit cell has been determined asthree phenylenes having a zig-zag conformation, with the meta-phenylenelying on the backbone plane. PEKK(T/I) shows crystalline forms I and II(as in the case of PEKK(T)) and also shows, under certain conditions, aForm III.

Suitable polyetherketoneketones are available from several commercialsources under various brand names. For example, polyetherketoneketonesare sold under the brand name OXPEKK™ polymers by Oxford PerformanceMaterials, Enfield, Conn., including OXPEKK™-C, OXPEKK™-CE, OXPEKK™-Dand OXPEKK™-SP polymers. Polyetherketoneketone polymers are alsomanufactured and supplied by Arkema. In addition to using polymers witha specific T:I ratio, mixtures of polyetherketoneketones may beemployed.

Other useful polymers in the present teachings include, but are notrestricted to PEEKEK, PEEKK, PEKEKK (where the E=ether and theK=ketone). Blends or mixtures of polyetherketoneketones may also beemployed within the scope of this invention. Other polymorphic polymersthat could benefit from the heat-treatment of the invention include, butare not limited to: polyamide 11 (PA11) and polyvinylidene fluoride(PVDF) homopolymers and copolymers.

The heat treatment described in the present teachings could also beapplicable to materials with a single crystal form such as PEEK(polyether ether ketone) and PEK (polyether ketone), where the treatmentat elevated temperatures will promote increases in the linear degree ofcrystallinity of the crystalline lamellae, affecting in a direct mannerthe melting temperature of the final product.

The powders used in these applications are prepared by a variety ofprocesses such as grinding, air milling, spray drying, freeze-drying, ordirect melt processing to fine powders. The heat treatment can beaccomplished before or after the powders are produced, but if treatedprior to forming the powders, the temperature of the powder formingprocess must be regulated so as to not significantly reduce the meltingtemperature or the quantity of the crystallinity formed in the heattreatment process.

The heat treatment process and the powders produced by this process arenot limited to any particular particle size. And as discussed above, theparticle size can be adjusted prior to or after the heat treatmentprocess based on the needs of the specific application as long as thebeneficial properties of high melting point and high degree ofcrystallinity are not compromised. In general, heat-treated powders willhave a weight average particle size of between 0.002 nm to 0.1 meter,and more preferably from 0.01 nm to 1.0 mm. For use in selective lasersintering (SLS) a weight average particle size of 15 to 150 microns ispreferred.

According to one aspect of the present teachings, a method for preparinga PEKK powder for use in SLS includes the steps of: providing a raw,non-powder PEKK material; heat treating the raw PEKK to evaporate atleast substantially all of a liquid solvent in the raw PEKK, causing atleast substantially all of the raw PEKK to be in the form ofirregularly-shaped particles; cooling the raw PEKK; and grinding the rawPEKK to form a fine PEKK powder.

Method of Laser Printing

In reference to FIG. 1, an SLS machine 10 in accordance with the presentteachings is illustrated. The system 10 includes a first chamber 20having an actuatable piston 24 disposed therein. A bed or base plate 22is disposed at an end of the piston 24. The temperature of the bed 22can be variably controlled via a controller 60 in communication withheating elements in and or around the bed 22. The bed temperaturesetpoint is input through an interface 62 into the controller 60.Software executing on the controller 60 transmits a signal to one ormore heating elements to heat the bed at or around the temperaturesetpoint. A second chamber 30 is adjacent to the first chamber 20. Thesecond chamber 30 includes a table surface 32 disposed on an end of apiston 34 disposed therein. A powder 36 for use during in the SLSmachine 10 is stored in the second chamber 30 prior to the sinteringstep.

While a particular embodiment of an SLS machine is disclosed, thepresent teachings are not limited in this regarding and the presentteachings may be practiced with different high temperature SLS machines,such as the EOSINT 800 manufactured and sold by EOS GmbH, based inKrailling, Germany.

Referring to FIG. 1, during operation of the SLS machine 10, a spreader40 or similar device traverses or translates across a top surface of thefirst chamber 20, evenly distributing a layer of powder 36 (e.g., PEKKpowder, fine PEKK powder, recycled PEKK powder) across either the topsurface of the bed 22, or the material previously disposed on the bed.The SLS machine 10 preheats the powder material 26 disposed on the bed22 to a temperature proximate to a melting point of the powder.Preheating is typically accomplished by heating the actual bed, asdescribed above, which transfers energy to the powder in the form ofheat via thermal conduction. Preheating the powder makes it easier forthe laser to raise the temperature of powder to a fusing point. In someembodiments of the present teachings, there are also one or more heatingelements in and/or around the second chamber 30 in order to heat thepowder 36 prior to being delivered to the sintering surface (bed 28).

A laser 50 and a scanning device 54 are disposed above the bed. In the P800, the laser is a 50 W CO₂ laser. It is noted that the 50 W CO₂ laseris exemplary, and the present teachings are not limited thereto. Thelaser transmits a beam 52 to the scanner 54, which then distributes alaser beam 56 across the layer of powder 36 disposed on the bed 22 inaccordance with a build program. The laser selectively fuses powderedmaterial by scanning cross-sections generated from a three dimensionaldigital description of the part on the surface of the bed 56 having alayer of the powdered material disposed thereon. The laser 50 and thescanner 54 are in communication with the controller 60. After across-section is scanned, the bed 22 is lowered by one layer thickness,a new layer of powdered material is disposed on the bed via the spreader40, and the bed is rescanned by the laser. This process is repeateduntil the build 28 is completed. During this process, the cylinder 34 inthe second chamber is incrementally raised to ensure that there issufficient supply of powder. The SLS machine in FIG. 1 may also beconfigured with a mechanism for analytically determining bedtemperatures of the SLS machine, as disclosed in U.S. application Ser.No. 14/472,817, the contents of which are incorporated herein byreference in its entirety for all purposes.

Using the above described PEKK, the bed temperature is set toapproximately 285 degrees Celsius and the laser power is set at a powerin the range of 1 W to 20 W. In some embodiments, the laser power is setwithin a range of 5 W to 10 W. In some embodiments, the laser power isset within a range of 6 W to 9 W. For example, the power may be set at 5W or 8 W. A powder layer thickness of 125 microns is typical. After thelayer-wise build is performed, the powder cake is allowed to cool atcontrolled rates. For example, for PEKK, the cake is typically cooled atbetween 1 and 100 degrees Celsius per hour. It should be appreciated bya person of ordinary skill in the art that the rate of cooling dependson the dimensions of the cake, with deeper beds typically requiring moretime to cool.

Parts manufactured by using the disclosed process demonstrated a greatertensile strength as determined by the ASTM D638 as compared to partsmanufactured using a laser power selected in the range of 30 W to 50 W,as is known in the art and as is recommended by the manufacturer of theEOSINT P 800 SLS machine.

In order to determine the optimal laser power to use in configuring theSLS machine for selectively laser sintering a particular batch ofpolymer powder, a method of analytically determining laser power forlaser printing may be used and is described in detail below.

Method of Analytically Determining Laser Power for Laser Printing

In one embodiment of the present teachings, a method of determining apreferred and/or optimal laser power for a specific batch or lot ofpowder is provided.

Referring to FIG. 2, the method first includes the step 102 of choosinga batch of powder material for building a plurality of test rods or testcoupons. A manufacturer specializing in additive manufacturing (e.g.,SLS) usually receives the base material from third-party producers indifferent batches. Each batch of base material is designated with abatch number and/or lot number, which identifies the origin andcharacteristics of the base material. The batch number and/or lot numberallows the history of the batch's production to be traced and providesfor quality control checks. Also, all relevant issues of control andproduction particulars may also be traceable from the batch and/or lotnumbers.

In some embodiments, the batch of powder comprises polymer powdermaterial. For example, the polymer powder comprises PAEK powder, and inparticular PEKK powder. The batch of powder may comprise PEKK powder oralternatively, PEKK polymer with carbon (ESD) that has been treatedprior to laser sintering. Still, in some embodiments, the batch ofpowder is made up of recycled polymer powder that were not subjected tofusion and were left-over from one or more previous build runs. Forexample, the recycled powder material may be left-over, unfused materialfrom one previous build (“Cake A”). Unfused powder material left-overafter having gone through two previous builds (“Cake B”) may be recycledand used. Still in other cases, the recycled powder material is unfusedmaterial left-over after having gone through three or more previousbuilds (e.g., “Cake C”, “Cake D”).

Once a batch has been chosen, a range of laser powers for sintering thepowder material is selected in step 104. For example, the range of laserpowers to test may be between 1 W and 20 W. In some embodiments, therange of laser powers to test may be narrower than 1 W to 20 W. That is,5 W to 15 W, 5 W to 10 W, or 6 W to 9 W may be possible laser powerranges. It is to be understood that the disclosed ranges are exemplaryand are not limited thereto.

Thereafter, in step 106, a plurality of power increments within theselected range is determined to define laser power settings forconstructing test rods (e.g., dumbbell structure, dogbone structure).For example, increments of 0.5 W may be determined for a range of 5 W to15 W, such that one or more test rods are constructed from lasersintering the polymer powder at each of 5.0 W, 5.5 W, 6.0 W, 6.5 W, 7.0W, . . . 14.0 W, 14.5 W, and 15.0 W. Depending on how wide or narrow theselected range of laser power is, larger increments (e.g., 1.0 Wincrement) may be used so that the number of test rods to be constructedand tested may be reduced. Conversely, smaller increments (e.g., 0.2 Wincrement, 0.1 W increment) may be determined in order to achievegreater precision and accuracy for determining which particular laserpower setting is preferable and/or optimal for sintering the chosenbatch of powder material. In one embodiment, at least 8 different laserpower settings are chosen for qualification testing. In a preferredembodiment, with respect to steps 104-106, a table with every past lotof powder that includes the range of exposures used, the temperaturesettings (of the chamber), the chosen exposure, and information from DSC(differential scanning calorimetry) can be used to determine the atleast 8 different laser power settings for the qualification build.

The method shown in FIG. 2 includes the step 108 of programming theadditive manufacturing machine (e.g., SLS machine) to construct a numberof identically shaped test rods at each laser power setting using thechosen batch of powder material. The step 108 may comprise programmingthe SLS machine to construct one or multiple test rods at each powersetting using the chosen batch of powder material. As such, the laser 50is adjusted in real-time to sinter at a first power setting (e.g., 5.0W) and subsequently sinter at a second power setting (e.g., 5.5 W), soon and so forth. FIG. 3 illustrates this particular configuration, wheremultiple test rods 202-216 are constructed using different laserwattages (depicted by different colors in the figure). For example, testrods 202 a-202 e are built using laser wattage at 5.0 W, while test rods204 a-204 e are built using laser wattage at 6.0 W. Further, the SLSmachine may be programmed to construct a plurality of test rods so thattheir longitudinal axes are aligned in the z-direction (test rods202-212) and another plurality of test rods so that their longitudinalaxes lie in or are parallel to the x-y plane. The two differentorientations for building the test rods allows for multiple tensilestrength tests, as will be described in detail below.

With multiple test rods being built at each laser power setting, onetest rod can be subjected to Z strength testing, while another test rodis subjected to X-strength testing, according to ASTM D638. Multipletest rods built at a given laser power also provide for multiple tensilestrength tests to be performed for that particular laser power. That is,two different test rods built at, for example 10 W, can be tested for Zstrength. The multiple tests can be used for purposes of verifyingresults or averaging results. Since the ASTM D638 testing processspecifies at least five specimens for testing, it is preferable that atleast five test rods at each laser power increment is constructed.

In some embodiments, the step of programming the SLS machine toconstruct the test rods involves programming the SLS machine to createtest rods at each laser power setting in one single build run (FIG. 3).Accordingly, the SLS machine is able to adjust the power of the laserbeam at various points during one single build run. In otherembodiments, the step of programming the SLS machine to construct thetest rods involves multiple build runs to create test rods at each laserpower setting. For example, the SLS machine may be programmed toconstruct multiple test rods at a laser power of 5.0 W in a first build,multiple test rods at a laser power of 5.5 W in a second build, multipletest rods at a laser power of 6.0 W in a third build, so on and soforth. Accordingly, the SLS machine adjusts the power of the laser atthe completion of a build. Alternatively, the SLS machine may beprogrammed to construct a first set, second set, third set, etc. of testrods in a first, second, third, etc. build, respectively, wherein eachset comprises at least one test rod constructed at each laser powersetting.

The method according to the present teachings then includes the step 110of using the SLS machine to construct the plurality of identicallyshaped test rods from the chosen batch of powder material based on theprogramming performed in step 108. Each of the test rods is sintered bya specified laser power setting. For example, in one embodiment, tendifferent laser powers are used to manufacture ten test rods. In thisembodiment, for example, a laser power setting of 5.0 W is used for thefirst rod. The laser power increases in half Watt increments for eachsubsequent test rod. For example, a laser power setting of 5.5 W is usedfor the second rod, a laser power setting of 6 W is used for the thirdrod. In some high temperature machines, such as the EOSINT P 800, themachine is configured to provide variable laser power in a single buildlayer, wherein different laser powers can be used in discrete sectionsof the build layer. In this manner, it is possible to prepare aplurality of test rods using different laser powers from a single lot ofpowder in a single build.

After the build is complete, in step 112, each test rod is removed fromthe powder cake and then inspected (e.g., visually, using tomography, orother computer-based imaging) for voids or other defects in thesurface(s) of the test rod. Voids and/or other defects in the test rodindicate a potential issue with the specific laser power used toconstruct the respective test rod. The step 112 may further compriseinspecting a fracture point of each test rod formed during an ASTM D638test process. In addition, the porosity of each test rod may beinspected (e.g., visually, using tomography, or other computer-basedimaging). For example, optical coherence tomography (OCT) or othercomputer-based imaging may be used to determine porosity of each testrod. The system shown in FIG. 1 may comprise an OCT sensor or scannerthat provides in-situ, real-time subsurface visualization of the testrods as they are built. In some embodiments of the present invention,precision instruments are used to measure any voids identified duringinspection.

In some embodiments, step 112 may also take into account the inspectionof each test rod's mechanical properties and tensile behavior during anASTM D638 testing process. For example, such inspection analysis maycomprise monitoring the manner in which the test rod fractures orbreaks, where the fracture occurs, and the presence of banding orbowing. The method may further include the step of analyzing otherfactors affecting the laser sintering and the lifecycle of the builttest rod.

In addition to inspecting each test rod for voids, the method accordingto the present teachings may include the additional step 114 ofmeasuring the tensile strength of each test rod and comparing thetensile strength results to determine strength of each test rod. Inparticular, each test rod is subjected to the ASTM D638 test procedure.ASTM D638 is a common plastic strength specifications and covers thetensile properties of unreinforced and reinforced plastics. This testmethod uses standard “dumbbell” or “dogbone” shaped specimens. Auniversal testing machine (tensile testing machine) performs the test.The testing machine obtains information about the tested part, includingthe tensile strength. The higher the yield strength, the more preferablethe test rod.

In step 116 of the method according to the present teachings, it isdetermined, based on the inspection (step 112) and/or tensile strengthmeasurements (step 114) which rod among the plurality of test rodsexhibits desired mechanical properties. For example, a rod exhibitingthe absence of voids in its surface has better mechanical propertiesthan a rod having voids. In addition to the results of inspection oralternatively, high tensile strength balanced with other factors (e.g.,porosity) may be used for determining which rod among the plurality oftest rods possess acceptable/desirable mechanical properties. In someembodiments, this step includes analyzing the measurements of Z strengthor the break point (psi) along the z-axis for each test rod. It is knownthat Z strength reflects the strength of the internal bond between thelayers of an additive manufactured part and is an importantcharacteristic in additive manufacturing. Based on evaluation of thetensile strength test results, one or more rods at a particular laserpower may be identified as exhibiting higher “Z strength”.

In other embodiments, the evaluation of tensile strength may alsoinclude an analysis of the elastic modulus or Young's modulus (psi) ofeach test rod, as well as the elongation to break/fracture (%) of eachtest rod. Other factors that may be monitored and evaluated areelongation at yield, nominal strain at break (grip separation), secantmodulus of elasticity, and/or Poisson's Ratio.

In some embodiments, the step 116 may include evaluating each test rodwith respect to its “x strength” or break point (psi) along the x-axis.For example, a test rod which exhibits improved tensile strength mayexhibit a high Z strength and a high X strength.

In accordance with the present teachings, a plurality or rodsmanufactured at different laser powers from a single lot of powder orduring a single build are inspected for void formation and/or testedusing the ASTM D638. The test rod with the most desirable qualities isfor example the rod having no formation of voids in the x-axis and ahigh Z strength.

Step 118 of the method according to the present teachings comprisesidentifying the laser power setting used to construct a respective testrod without formation of voids in the surface as an optimal laser power.In some embodiments, the optimal laser power is at least one watt belowa lowest laser power setting associated with the formation of voids.That is, an exposure that is at least one watt below any voids in thex-bar or x-axis results. In other embodiments, the optimal laser poweris at least one watt below a lowest laser power setting associated withthe formation of voids and exhibits z-axis tensile strength that has notdecreased substantially. In general, the highest wattage while avoidingpossible voids is considered the optimal and/or preferable. In yet otherembodiments, the step of identifying the optimal laser power comprisesselecting a laser power setting based on the absence of voids in therespective test rod and on the tensile strength of the respective rod,wherein the optimal laser power provides the highest strength in z-axiswith no formation of voids.

Thereafter, the laser power associated with the selected rod is thenused for subsequent builds made from the same batch/lot of powder (step120). That is, the SLS machine may be configured to use theoptimal/preferable laser power when building an object(s) with powderfrom the chosen batch. In some embodiments, the optimal/preferable laserpower and corresponding batch/lot number may be saved in a database ormemory storage unit within the controller 62 so that the SLS machine canbe configured at a later time when future construction of an objectusing the chosen batch of powder material is required.

In some embodiments of the present teachings, a DSC analysis of thespecific lot of powder is performed and stored in association with theoptimal laser power. In this manner, it is possible to prepare a libraryof optimal laser powers for different lots of powders based on the DSC.Such library may be saved in a database or memory storage unit withinthe controller 62. Thus, when a new lot of powder is prepared, a DSCanalysis is conducted. If the DSC profile is similar to a profile storedin the library, the operator can determine the laser associated with thesimilar DSC profile in the library and use it with the new lot ofpowder.

Finally, the method shown in FIG. 2 may comprise a step 130, whereinsteps 104-120 are repeated for another batch of powdered material.

FIG. 4 shows a table of exemplary lots of powder material and theoptimal and/or preferable laser power that was determined to produceobjects with no voids and strong mechanical properties for therespective lot. For Lot #300393, which consisted of Virgin ESDPEKK+Fines, an optimal laser power of 13.5 W was determined using themethod according to the present teachings. In contrast, a laser power of13.0 W was determined to be optimal or preferable for Lot #7215consisting of Virgin ESD PEKK. It was also unexpectedly discovered thatrecycled polymer powder demonstrated even lower laser powers to achieveno voids and strong tensile properties in objects constructed therefrom.For example, Lot #300652 represents Cake A ESD (First Recycle of PEKKpowder) and has an optimal laser power of 8.0 W. Moreover, a laser powerof 5.0 W was determined as being optimal for Lot #300595, consisting ofCake B ESD (Second Recycle of PEKK powder).

FIGS. 5a-5d shows detailed results of the inspection (forpresence/absence of voids) and tensile strength tests (ASTM D638)performed on the tests rods produced using powdered material from thelots identified in FIG. 4. In FIG. 5a , relating to Lot No. 300393Virgin ESD with fines, the lowest laser power setting associated withthe formation of voids in the test rod was 14.5 W. Thus, the laser powersetting of 13.5 W, which is one watt below 14.5 W and which exhibited noformation of voids, was identified as being the optimal laser power. Thetest rod produced using the 13.5 W laser beam exhibited a break point ofapproximately 10680 psi in the z-axis.

In FIG. 5b , relating to Lot No. 7215 Virgin ESD PEKK, the lowest laserpower setting associated with the formation of voids in the test rod was14.0 W. Thus, the laser power setting of 13.0 W, which is one watt below14.0 W and which exhibited no formation of voids, was identified asbeing the optimal laser power. The test rod produced using the 13.0 Wlaser beam exhibited a break point of approximately 10696 psi in thez-axis.

In FIG. 5c , relating to Lot No. 300652 represents Cake A ESD, thelowest laser power setting associated with the formation of voids in thetest rod was 9.0 W. Thus, the laser power setting of 8.0 W, which is onewatt below 9.0 W and which exhibited no formation of voids, wasidentified as being the optimal laser power. The test rod produced usingthe 8.0 W laser beam exhibited a break point of approximately 10375 psiin the z-axis.

In FIG. 5d , relating to Lot No. 300595 represents Cake B ESD, thelowest laser power setting associated with the formation of voids in thetest rod was 6.0 W. Thus, the laser power setting of 5.0 W, which is onewatt below 6.0 W and which exhibited no formation of voids, wasidentified as being the optimal laser power. The test rod produced usingthe 5.0 W laser beam exhibited a break point of approximately 10616 psiin the z-axis.

These results help demonstrate that recycled polymer powder materialprovides for lower laser power in laser sintering processes, withoutsubstantially reducing overall tensile strength and causing formation ofvoids in the built object/part.

FIGS. 6a-6d shows inspection results (regarding formation of voids) oftest rods in further detail. The data contained in FIGS. 6a-6d areassociated with the data contained in FIGS. 5a-5d , respectively. Theseinspection results were utilized in determining and identifying theoptimal laser power setting.

While the present teachings have been described above in terms ofspecific embodiments, it is to be understood that they are not limitedto those disclosed embodiments. Many modifications and other embodimentswill come to mind to those skilled in the art to which this pertains,and which are intended to be and are covered by both this disclosure andthe appended claims. It is intended that the scope of the presentteachings should be determined by proper interpretation and constructionof the appended claims and their legal equivalents, as understood bythose of skill in the art relying upon the disclosure in thisspecification and the attached drawings.

What is claimed is:
 1. A process for layer-by-layer manufacturing athree-dimensional object comprising the steps of: selecting a powdercomprising polyetherketoneketone (PEKK); constructing a plurality oftest rods from the selected powder using a laser by selective lasersintering (SLS), each of the plurality of test rods being sintered at adifferent laser power; inspecting each test rod for voids in a surfaceof the test rod, the voids having a width of at least 0.005″;identifying a laser power L used to construct a test rod in theplurality of test rods without formation of voids having a width of atleast 0.005″ in a surface of the test rod as an optimal laser power forthe selected powder; applying a layer of the selected powder on a bed ofa laser sintering machine; sintering selected points of the appliedlayer of the selected powder by irradiation using heat energy introducedby a laser having the laser power L; and successively repeating the stepof applying and the step of sintering until all cross sections of thethree-dimensional object are sintered.
 2. The process of claim 1,wherein the powder comprises unused PEKK that has not been used in aselective laser sintering (SLS) process.
 3. The process of claim 2,wherein the laser power L is between 10 Watts (W) and 20 W.
 4. Theprocess of claim 3, wherein the laser power L is between 10 W and 15 W.5. The process of claim 1, wherein the powder comprises a recycled PEKKpowder, wherein the recycled PEKK is a PEKK powder that has beenpreviously used in a selective laser sintering (SLS) process.
 6. Theprocess of claim 5, wherein the laser power L is between 1 W and 10 W.7. The process of claim 6, wherein the recycled PEKK powder comprisesCake A, wherein Cake A is a PEKK powder that has been subject to onlyone prior SLS process.
 8. The process of claim 7, wherein the laserpower L is between 5 W and 10 W.
 9. The process of claim 6, wherein therecycled PEKK powder comprises Cake B, wherein Cake B is a PEKK powderthat has been subject to only two prior SLS processes.
 10. The processof claim 6, wherein the powder comprises carbon fiber.
 11. The processof claim 10, wherein an average in-plane tensile strength of thethree-dimensional object is greater than 10 ksi.
 12. The process ofclaim 11, wherein an average out-of-plane tensile strength of thethree-dimensional object is greater than 10 ksi.
 13. A three-dimensionalobject made by a process comprising the following steps: selecting apowder comprising polyetherketoneketone (PEKK); constructing a pluralityof test rods from the selected powder using a laser by selective lasersintering (SLS), each of the test rods of the plurality of test rodsbeing sintered at a different laser power; inspecting each test rod forvoids in a surface of the test rod, the voids having a width of at least0.005″; identifying a laser power L used to construct a test rod in theplurality of test rods without formation of voids having a width of atleast 0.005″ in a surface of the test rod as an optimal laser power forthe selected powder; applying a layer of the selected powder on a bed ofa laser sintering machine; sintering selected points of the appliedlayer of the selected powder by irradiation using heat energy introducedby a laser having the laser power L; and successively repeating the stepof applying and the step of sintering until all cross sections of thethree-dimensional object are sintered.
 14. The three-dimensional objectof claim 13, wherein the powder comprises unused PEKK, wherein theunused PEKK is a PEKK powder that has not been used in a selective lasersintering (SLS) process.
 15. The three-dimensional object of claim 14,wherein the laser power L is between 10 Watts (W) and 20 W.
 16. Thethree-dimensional object of claim 15, wherein the laser power L isbetween 10 W and 15 W.
 17. The three-dimensional object of claim 13,wherein the powder comprises a recycled PEKK powder, wherein therecycled PEKK is a PEKK powder that has been previously used in aselective laser sintering (SLS) process.
 18. The three-dimensionalobject of claim 13, wherein the laser power L is between 1 W and 10 W.19. The process of claim 1, wherein the laser power L is between 1 W and20 W.
 20. The three-dimensional object of claim 13, wherein the laserpower L is between 1 W and 20 W.